Motion sensors, which include gyroscopes and their components (e.g., angular rate sensors and accelerometers), are widely used in consumer electronics products such as VCR cameras, and in aerospace and automotive applications such as safety control systems and navigational systems. Examples of automotive applications for gyroscopes include traction control, ride stabilization and global position systems. Electromechanical and electronic motion sensors have been widely used in the automotive industry to detect an automobile's deceleration. More recently, sensors that employ an electrically-conductive, micromachined plated metal or silicon sensing element have been developed which can be integrated with bipolar/CMOS/BiCMOS circuits on a silicon wafer.
An example of a plated metal surface micromachine is disclosed in U.S. Pat. No. 5,450,751 to Putty et al., assigned to the assignee of this invention. The disclosed micromachine is formed by a metal plating technique in cooperation with a mold that defines the shape of the micromachine on the surface of a wafer. Putty et al. further disclose a novel configuration for the micromachine, which includes a resonating metal ring and spring system. Two embodiments of Putty's sensor are shown in FIGS. 1, 2 and 3. The sensor 10 shown in FIGS. 1 and 2 includes a ring 14 that is supported by a number of arcuate springs 16 radially extending from a center post or hub 18, all of which are formed on a sensing wafer 12. The ring 14 is surrounded by a number of equi-angularly spaced electrode structures 20 formed on the wafer 12 in close proximity to the perimeter of the ring 14. The ring 14 and electrode structures 20 are electrically conductive, so that the ring 14 and electrodes 20 are capacitively coupled. The sensor 30 of FIG. 3 is similar to that of FIGS. 1 and 2, with a ring 34 supported by a number of arcuate springs 36 from a central hub 32. The difference between the two embodiments is the shape of the springs 16 and 36. The springs 16 of FIG. 1 are C-shaped, essentially semicircular with a constant radius of curvature. In contrast, the springs 36 of FIG. 3 are S-shaped, essentially formed by two C-shaped portions having equal radii of curvature.
A variation of the sensor disclosed by Putty et al. is described in U.S. Pat. No. 5,547,093 to Sparks, which teaches an electrically-conductive, micromachined silicon sensing element formed by etching a single-crystal silicon wafer or a polysilicon film on a silicon or glass handle wafer. A sensor disclosed in U.S. Pat. No. 5,872,313 to Zarabadi et al. is also based on Putty et al., but has a sensing ring and electrodes with interdigitized members. The positions of the interdigitized members relative to each other enable at least partial cancellation of the effect of differential thermal expansion of the ring and electrodes, reducing the sensitivity to temperature variations in the operating environment of the sensor.
All of the above sensors operate on the basis of capacitively sensing movement of their rings toward and away from their sensing electrodes. More particularly, referring to the embodiment of FIGS. 1 and 2, some of the electrode structures 20 operate as drive electrodes to drive the ring 14 into resonance, while other electrode structures 20 are configured as sensing electrodes to capacitively sense the proximity of the ring 14, which will vary due to Coriolis forces that occur when the resonating ring 14 is subjected to rotary motion. In FIG. 1, eight electrode structures 20 are equi-angular spaced along the perimeter of the ring 14, so that adjacent structures 20 are positioned forty-five degrees apart from each other. The drive electrodes induce two identical elliptically-shaped vibration modes in the ring 14 to sense ring rotation, or angular rate. One of the elliptically-shaped modes, the primary mode, is driven electrostatically by the drive electrodes. In the elliptical vibration modes of the ring 14, only tangential deflection of the ring 14 occurs at the nodes (i.e., radial motion is zero), while only radial deflection occurs at what are termed the antinodes (i.e., tangential motion is zero). In an ideal ring, the nodes and antinodes are spaced forty-five degrees apart; hence, the reason for spacing the electrode structures 20 forty-five degrees apart as shown in FIGS. 1 and 3. Sensing electrodes are positioned adjacent the four nodes of the ring 14 to capacitively sense the radial and tangential motion of the ring 14. If the resonating ring 14 is not subject to any rotation, capacitance between the ring 14 and the sensing electrodes next to the nodes will not change since there is no radial motion at the nodes in the primary vibration mode, and therefore no rate signal. However, when the ring 14 is subjected to rotation, or angular rate, the Coriolis force will transfer energy from the primary vibration mode to the second vibration mode, which is forty-five degrees apart from the primary mode. The deflection of the second vibration mode changes the gap between the ring 14 and the sensing electrodes at the nodes, changing the capacitance and generating a rate signal.
Sensors of the type described above are capable of extremely precise measurements, and are therefore desirable for use in automotive applications. However, further research has shown that mode shapes of sensing rings supported by springs configured as shown in FIGS. 1 and 3 deviate from the symmetrically elliptical mode shape of an ideal ring. Mode shape distortion causes the nodes to shift and, in the case of the sensor taught by Zarabadi et al., has been found to induce a rocking motion in the interdigitized features at the antinodes, which limits the performance of the sensor and can lead to sticking of the interdigitized features.
The impact of node shift on sensor performance can be seen from a comparison of FIGS. 4 and 5, which are maximum deflection plots of an ideal ring at flexural mode (based on finite element analysis (FEA)) and the sensor ring 14 of Putty et al. As seen from FIG. 4, the ideal ring has only radial deflection (identified as UR) at the antinodes (0, 90, 180 and 270 degrees) and only tangential deflection (identified as UTHETA) at the nodes (45, 135, 225 and 315 degrees), as discussed above. The node shift observed in FIG. 5 is the result of the symmetrical elliptical mode shapes of the ideal ring being greatly distorted by the springs 16 of the sensor 10. The antinodes (where no tangential deflection occurs) and the nodes (where no radial deflection occurs) are no longer forty-five degrees apart. Consequently, the nodes are not aligned with the sensing electrodes at the 45, 135, 225 and 315-degree positions around the ring 14, with the result that radial deflection of the ring 14 occurs at the sensing electrodes and a capacitance change or offset is sensed by the sensing electrodes even though the gyroscope is not subjected to angular rotation. If a balance mechanism is used to align the nodes with the sensing electrodes located at 45, 135, 225 and 315 degrees, the antinodes will be shifted away, and sensing electrodes located at 0, 90, 180 and 270 degrees will see tangential motion in addition to radial motion. This additional motion distorts the signal picked up at the electrodes, increases the 2.times. frequency signal and total harmonic distortion (THD), and makes the task of calibrating the gyroscope difficult. Finally, and as noted above, if the interdigitized features of Zarabadi's sensor are present, the tangential motion of the ring 14 can also cause the features to rock and stick.
Therefore, it would be highly desirable if further advancements could be made toward improving the performance and durability of motion sensors having resonance ring gyroscopes of the type described above.